In the detection and amplification of low level signals derived from a high impedance source, the first stage of an amplifier may attenuate the incoming signal because of low input impedance or may introduce an input signal component related to the output, causing feedback oscillation. While various techniques have been proposed to circumvent these problems, those techniques are difficult to implement, especially in applications requiring the use of a low supply voltage, such as the single power cell usually used in a hearing aid amplifier. The causes and results of these problems are explained below.